Submitted to: Bioresource Technology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/14/2013
Publication Date: 1/6/2014
Publication URL: http://handle.nal.usda.gov/10113/59561
Citation: Piazza, G.J., Lora, J., Garcia, R.A. 2014. Flocculation of high purity wheat straw soda lignin. Bioresource Technology. 152:548-551. DOI: org/10.1016/j.biortech.2013.11.040. Interpretive Summary: About one-third of biomass is composed of lignin, and because of this lignin is the major byproduct of biorefineries. The lignin is used in some industrial and consumer applications, and lignin can be converted to materials which are substitutes for petrochemicals. Purification of the lignin for subsequent use is laborious and requires extraction into strongly basic solutions, and precipitation of the lignin with strong inorganic acid. The presence of the acid on the recovered lignin requires the lignin to be extensively washed, and much lignin is lost in the washing step. An improved route for lignin isolation may involve the use of flocculants which can remove the lignin from solution without the need for strong acid. High concentration lignin solutions were prepared, and these were treated with five different flocculant materials. Two flocculants cause most of the lignin to be recovered from solution: Poly(diallydimethylammonium chloride and bovine blood, a byproduct of meat production. Various tests were performed to determine the optimal amount of flocculant to use and the degree to which lignin could be isolated. Under the best conditions 90% of the lignin could be recovered.
Technical Abstract: Flocculant action on lignocellulose mixtures has been studied, but flocculant action on purified sulfur-free lignin has not been reported. In the last step of the industrial process, the purified lignin solution is acidified with sulfuric acid which causes the lignin to become insoluble. The feasibility of using flocculant for the isolation of lignin was tested on pH 5.5 lignin solutions. A high concentration of lignin was used to mimic the conditions found in production facilities. Poly(diallydimethylammonium chloride) (pDADMAC) and bovine blood (BB) caused most of the lignin to become insoluble while treatment by cationic polyacrylamide, chitosan, and the soy protein PF 974 removed only a portion of lignin from solution. Alum (aluminum sulfate) also caused lignin insolubility. Subsequent measurements were made with PDDAC and BB with and without alum. Turbidity measurements showed different optimal concentration for pDADMAC and BB, but the higher magnitude of BB-induced turbidity did not correspond to greater lignin flocculation. Zeta potential of lignin mixtures was negative, and as pDADMAC was added the Zeta potential decreased and eventually became positive. The pDADMAC/lignin ratio at zero Zeta potential was close but higher than the optimal level of this ratio for lignin flocculation. The Zeta potential with BB was always negative, and the plot of Zeta potential versus the BB/lignin formed a plateau at higher ratio values. Since the insoluble lignin did not gravity sediment, the flocculant-lignin mixtures were centrifuged. Dry mass weight and spectroscopic determination of optimal pDADMAC and BB levels were in good agreement. Spectroscopic results were corrected for the absorption of BB, and gave the percent of isolated lignin as 87-92%. BB in the supernatant and pellet was determined with nitrogen analysis, and it was found that BB concentrated in the pellet until very high amounts of BB were added. Ash mass was also determined. Lignin concentrations in BB trials were directly determined by subtracting ash and BB mass from pellet and supernatant dry mass, and in spite of high error level in the results, the values for optimal BB were close to those from other determinations. Alum caused increased lignin flocculation at lower levels of pDADMAC and BB, but alum had an insignificant effect on the optimal level of these flocculants.